Metal-oxide-semiconductor field-effect transistors (MOSFET) are widely used semiconductor devices. The escalating need for increased densification and performance in ultra-large scale integration semiconductor devices requires reduced sized MOSFETs.
The structure of a typical MOSFET is shown in FIG. 1. Referring to FIG. 1, the structure shown comprises a semiconductor substrate 10 having an active region isolated by surrounding field oxide region 1. The active region comprises source/drain regions 12 and 13, and gate electrode 14 spaced apart from semiconductor substrate 10 by gate dielectric layer 15. Source drain regions 12 and 13 are characterized by shallow sub-regions 17A and deep sub-regions 17B. Deep sub-regions 17B and shallow sub-regions 17A may or may not have substantially the same dopant concentrations. Sidewall spacers 16 are formed on the side surfaces of gate electrode 14. The channel region between shallow sub-regions 17A is designated by reference numeral 19.
In a conventional method for forming a MOSFET structure shown in FIG. 1, two ion implantation steps are performed. One ion implantation is performed before formation of the sidewall spacers 16 to form shallow sub-region 17A and another is performed after formation of the sidewall spacers to form deep sub-region 17B. During the ion implantation to form the deep sub-regions 17B, the sidewall spacers 16 shield the underlying semiconductor substrate, thereby limiting dopant penetration into shallow subregion 17A.
As the size of a MOSFET structure shrinks into the sub-micron range, the depth of the shallow regions falls into the ultra-shallow range, (i.e. below 0.1 micron). Producing shallow subregions that fall into the ultra-shallow range through conventional implantation is problematic.
Producing shallow subregions of ultra-shallow depth using ion implantation requires low energy implantation. Low energy implantation typically uses energy levels below 10 keV.
Much of the implantation equipment found in semiconductor manufacturing facilities does not operate at these low energy levels. While implantation equipment capable of generating low energy ion beams is available, the equipment may have to be purchased. Obviously, purchasing additional equipment drives up the cost of manufacturing. Additionally, this equipment tends to be relatively inefficient compared to higher energy implantation equipment, thus further raising the cost of manufacturing.
Furthermore, forming shallow subregions using low energy level implantation has several problems. As one skilled in the art is aware, implantation is usually followed by activation annealing at some point in the production of MOSFETs. As a result of crystalline defects, such as those induced by ion implantation, implanted impurities rapidly diffuse, through a process known as transient enhanced diffusion, during activation annealing thereby expanding the extent of the doped region. This expansion renders it extremely difficult to achieve shallow junction regions.
Naturally forming surface films, such as native oxides, that form on the surfaces of the area to be implanted are especially troublesome to low energy level implantation. Typically, naturally forming surface films are so thin that they have a negligible affect on implantation. However, as the targeted junction depths become shallower and the implantation energies become lower, variations in the thickness of naturally forming surface films become very significant. At the low energies necessary to create shallow subregions by implantation, variations in the native oxide layers adversely affect the quality of shallow regions that can be formed through low energy implantation.
There exists a need to provide a method for forming shallow subregions that avoids the cost attendant upon implantation equipment capable of low energy level implantation. There also exists a need to provide a method that can be used to form ultra shallow regions that avoids problems attendant to implantation at low energies, e.g., transient enhanced diffusion and variation in the thickness of naturally forming surface film.